This invention relates to a method and apparatus for determining whether a pipe joint is properly connected. More particularly, it relates to a method and apparatus for determining the tightened condition of a pipe joint for connecting sections of oil well pipe.
Pipe joints for oil well pipes must be able to form a joint capable of resisting the high pressures which develop inside the pipes. From in the past, a threaded pipe joint like that shown in FIG. 4 which creates a seal by metal-to-metal contact has been used to connect oil well pipe.
The pipe coupling 1 is a cylindrical member having female threads 10,10 formed at both ends for prescribed lengths on its inner periphery. Adjoining the female threads 10,10 are tapered seal surfaces 11,11 which decrease in diameter towards the lengthwise center of the pipe coupling 1. Shoulder surfaces 12,12 which adjoin the seal surfaces 11,11 extend roughly perpendicular to the longitudinal axis of the pipe coupling 1. A pipe 2 to be connected to this pipe coupling 1 has at its end portion an end surface 22 which is roughly perpendicular to the longitudinal axis of the pipe 2, a seal surface 21 which adjoins the outer periphery of the end surface 22 and has a taper corresponding to the seal surfaces 11 of the pipe coupling 1, and a male thread 20 which adjoins the seal surface 21 and is shaped to engage with the female threads 10 of the pipe coupling 1. The joining of the pipe coupling 1 and the pipe 2 is performed by placing the inside of the pipe coupling 1 over the end of the pipe 2 and then rotating the pipe coupling 1 about its longitudinal axis to screw the male threads 20 of the pipe 2 into the female threads 10 of the pipe coupling 1. FIG. 5 is an enlarged view of an assembled joint. As shown in this figure, when the coupling 1 and the pipe 2 are properly joined, the seal surface 11 on the inner periphery of the pipe coupling 1 and the seal surface 21 on the outer periphery of the pipe 2 are in intimate contact along their entire lengths to form a cylindrical seal. Furthermore, the shoulder surface 12 of the pipe coupling 1 abuts against the end surface 22 of the pipe 2 and is in intimate contact therewith to form a shoulder seal. Due to both seals, the joint has a high sealing integrity.
The joining of the pipe coupling 1 to the pipe 2 is carried out by a tightening machine equipped with a rotating chuck that holds the pipe coupling 1 and rotates it about its longitudinal axis and a fixed chuck that secures the pipe 2 on the same axis. As the coupling 1 and the pipe 2 are being screwed together, the tightening torque which is applied to the pipe coupling 1 through the rotating chuck is detected, and when the detected value reaches a prescribed final tightening torque level, the operation of the tightening machine is stopped, and the joining of the coupling 1 and the pipe 2 is completed. As a result, the pipe coupling 1 and the pipe 2 are fastened within a tightening torque within a prescribed range of the final tightening torque.
FIG. 6 is a graph showing the variation of the tightening torque applied to the pipe coupling 1 during joining. In this figure, the horizontal axis indicates the number of turns of the pipe coupling 1. When tightening is properly carried out, as shown in the figure, the tightening torque at first gradually increases with an increase in the number of turns, and midway through it rapidly increases towards the final tightening torque. The transition from the gradual increase region to the rapid increase region occurs when the shoulder surface 12 of the pipe coupling 1 abuts against the end surface 22 of the pipe 2. Thus, the tightening torque applied to the pipe coupling 1 during the region of gradual increase is mainly consumed in overcoming the frictional resistance between the female threads 10 and the male threads 20 and between seal surface 11 and seal surface 21. In contrast, in the region of rapid increase, the tightening torque is mainly consumed in pressing the shoulder surface 12 against the end surface 22.
Accordingly, by determining whether the coordinates of the transition point A from the gradual increase region to the rapid increase region are suitable, and in particular, by determining whether the number of turns of the pipe coupling 1 and the tightening torque at transition point A are both within appropriate ranges, it can be determined whether the joint is properly tightened.
For example, in the process of tightening, if galling takes place between the female threads 10 and the male threads 20, at the time of galling, the tightening torque will rapidly increase, so the torque will vary as shown by the dashed line in FIG. 6. On the other hand, when the tightening between the female threads 10 and the male threads 20 and between seal 11 and seal 21 is larger than appropriate, the frictional resistance will be large, so the slope will be large in the region of gradual increase. Therefore, the torque will vary as shown by the long and short dashed line in FIG. 6. In either case, the final tightening torque is obtained before a suitable number of turns of the coupling 1 have taken place, and in the completed joint, there will be no contact or insufficient contact between the shoulder surface 12 of the coupling 1 and the end surface 22 of the pipe 1. Therefore, a desired seal will not be obtained. As shown in FIG. 6, if the joint is not properly tightened, the transition point will be in a location much different in the figure from the correct transition point A, so a bad joint can be recognized and rejected.
As described above, in general, the tightening torque and the number of turns during tightening are continuously sensed by a torque detector and a turn detector mounted on a joining machine. The sensed values are displayed in the form of a graph or a table, and a human operator watching the display determines whether the joint is properly made up or not. However, this arrangement has the problem that it requires a full-time operator, so it is uneconomical with respect to labor costs. Furthermore, the result of evaluation will differ from operator to operator, so consistent results can not be obtained.
In order to automate determination of the tightened condition of a pipe joint and thereby realize labor savings, in Japanese Published Unexamined Patent Application No. 63-229272, No.58-165972, and No.3-54430 a method is proposed in which values sensed by a torque detector are sampled at minute time intervals from the start of joining of a coupling and a pipe. The difference between the sampled torque at two points in time is computed in real time, and the transition point A is determined to have been reached when the difference exceeds a predetermined reference value. Based on whether the tightening torque at the transition point is within suitable ranges, the quality of the joint is determined.
However, in the gradual increase region prior to the transition point A, the tightening torque may fluctuate due to various disturbances, such as the entry of an impurity between the female threads 10 and the male threads 20. Therefore, the tightening torque may not increase smoothly and may fluctuate as shown in FIG. 6. Therefore, when the transition point is determined by the method described in Japanese Published Unexamined Patent Application No. 63-229272, for example, a small fluctuation in the torque curve in the gradual increase region may be mistaken for the transition point A, and the determination of the quality of the pipe joint would be incorrect. Furthermore, when the rotational speed varies during the tightening operation, the rate of change of the tightening torque with respect to time will also vary. So, the accuracy of the determination of the transition point by this method is poor. Therefore, the reliability of this method is extremely low.